What nerves are blocked in a subtenons (sub-Tenon's) injection?

Nerves Blocked in a Sub-Tenon's Injection

A sub-Tenon's injection blocks the ciliary nerves, including the short and long posterior ciliary nerves, as well as branches of the nasociliary nerve, effectively providing anesthesia to the anterior and posterior segments of the eye.

Anatomical Basis of Sub-Tenon's Block

Sub-Tenon's anesthesia involves the injection of local anesthetic into the sub-Tenon's space, which is the potential space between the Tenon's capsule and the sclera. This technique targets several key nerves:

1. Short posterior ciliary nerves: These nerves supply the:

   • Ciliary body
   • Iris
   • Cornea (via anterior ciliary nerves)

2. Long posterior ciliary nerves: These provide sensory innervation to:

   • Cornea
   • Iris
   • Ciliary body

3. Branches of the nasociliary nerve: These supply:

   • Sensory innervation to the anterior segment
   • Branches to the ciliary ganglion

Mechanism of Action

The local anesthetic injected into the sub-Tenon's space diffuses posteriorly around the globe to block:

• Sensory nerves (providing anesthesia)
• Motor nerves (providing akinesia when sufficient volume is used)
• Autonomic nerves (affecting pupillary responses)

The anesthetic solution bathes the sclera and blocks the nerve fibers as they pass through the sub-Tenon's space, effectively creating a complete sensory and motor block of the eye 1.

Clinical Effectiveness

Sub-Tenon's block has been shown to provide effective anesthesia for various ophthalmic procedures:

• For posterior segment surgery, a single injection of sub-Tenon's anesthesia is relatively safe and effective, particularly for vitrectomies lasting less than 3 hours 1
• For optic nerve sheath fenestration, sub-Tenon's local anesthesia provides excellent anesthesia and maximal patient comfort 2
• For cataract surgery, the technique provides satisfactory anesthesia, though total akinesia is achieved in only about 20% of patients 3

Advantages Over Other Techniques

Sub-Tenon's block offers several advantages over retrobulbar or peribulbar techniques:

• Reduced risk of globe perforation (no sharp needle penetration into the orbit)
• Lower risk of retrobulbar hemorrhage
• Avoidance of direct optic nerve injury
• Reduced risk of brainstem anesthesia

Potential Complications

While generally considered safer than needle-based blocks, sub-Tenon's injection can still cause complications:

• Chemosis (conjunctival edema)
• Subconjunctival hemorrhage
• Conjunctival entrapment of foreign material (rare) 4
• Scleral ulceration (rare) 4
• Orbital inflammation

Technical Considerations

The effectiveness of the block depends on:

1. Proper technique: Accessing the sub-Tenon's space through a small conjunctival incision, typically in the inferonasal quadrant
2. Appropriate anesthetic mixture: Often a combination of lidocaine and bupivacaine with hyaluronidase
3. Adequate volume: Usually 3-5ml for anterior segment procedures, and up to 11ml for posterior segment procedures 1

Sub-Tenon's block has become a popular alternative to retrobulbar and peribulbar blocks due to its favorable safety profile while still providing effective anesthesia by blocking the ciliary nerves and their branches as they course through the sub-Tenon's space.